Can be caused by several different things:
1) Something is actually close to you (or you are close to it!)
2) Exhaust fumes from the vehicle in front (yup, the sensors are ultrasonic and bendy hot air can distort the return signal)
3) Other vehicles with parking/distance sensors active on the road that interfere with yours. Its really annoying.
Of course, as has been said, you could turn off the sensors and just engage them when parking... I get annoyed with mine but still find them more useful on all the time.
